RAKE receivers are well known in the communication arts and find widespread use in Code Division Multiple Access (CDMA) systems, such as in IS-95, IS-2000 (cdma2000), and Wideband CDMA (WCDMA) wireless communication networks. The name derives from the rake-like appearance of such receivers, wherein multiple, parallel receiver fingers are used to receive multiple signal images in a received multipath signal. By coherently combining the finger outputs in a RAKE combiner, the conventional RAKE receiver can use multipath reception to improve the Signal-to-Noise Ratio (SNR) of the received multipath signal.
Generalized RAKE (G-RAKE), or chip level equalizer, is an enhancement of the traditional RAKE receiver in a CDMA system. To cope with sudden changes in the interference, it is highly desirable to have a G-RAKE at each time slot and not to rely on cross-slot average. The difficulty is the estimation of the impairment co-variance matrix, i.e. the so-called Ru matrix.
In uplink, non-parametric G-RAKE employs un-used Walsh codes to estimate the impairment co-variance matrix, which leads to extremely good performance close to an ideal G-RAKE. The Walsh code tree structure in uplink implies that there always exist a number of un-used Walsh codes and the network knows them.
Walsh Code is a group of spreading codes having good autocorrelation properties and poor crosscorrelation properties. Walsh codes are the backbone of CDMA systems and are used to develop the individual channels in CDMA. For IS-95, there are 64 codes available. Code 0 is used as the pilot and code 32 is used for synchronization. Codes 1 though 7 are used for control channels, and the remaining codes are available for traffic channels. Codes 2 through 7 are also available for traffic channels if they are not needed. For cdma2000, there exists a multitude of Walsh codes that vary in length to accommodate the different data rates and Spreading Factors of the different Radio Configurations.
The information of complete downlink Walsh code allocation is not available to User Equipment (UE), moreover Walsh codes might be fully occupied by High Speed Downlink Packet Access (HSDPA), Release 99 (R99) and control channels and left no available un-used Walsh codes. Thereby UE cannot rely on un-used Walsh codes to estimate the impairment co-variance matrix. Instead, it uses either Common Pilot Channel (CPiCH), which is a commonly used method but very noisy, or potentially some data covariance or data-aided methods. So far there has been no sufficiently robust product algorithm proposal developed.
In downlink, G-RAKE is primarily intended for the reception of High Speed-Downlink Shared Channel (HS-DSCH), which is always associated with a High Speed-Shared Control Channel (HS-SCCH). HS-SCCH carries scheduling information like: 1) UE ID; 2) HS-DSCH transport format including Walsh code set (starting node and number of codes), modulation scheme and transport block size; 3) hybrid-ARQ-related parameters. The timing relation between HS-SCCH and HS-DSCH ensures that information like Walsh code set and modulation scheme is available for HS-DSCH on-the-fly demodulation. The structure of HS-SCCH limits the scheduling information only available to the intended UE, i.e. a UE which is not scheduled has no access to this information.
Since un-used Walsh codes are not available in downlink, a practical solution would be to have a parametric G-RAKE based on CPiCH, but it would not perform very well especially at high geometry (>15 dB), or to build a non-parametric G-RAKE for low mobile speed in order to allow for cross-slot average, which becomes an issue when facing sudden changes of interference, or to build a Minimum Mean Square Error (MMSE) G-RAKE or chip-level equalizer, but it can have catastrophic performance at high Signal to Interference-plus-Noise Ratio (SINR) regions, especially for Higher Order Modulation (HOM) like 16QAM (Quadrature amplitude modulation).
In order to overcome the shortage of number of pilots, 10 symbols per time slot, in the impairment co-variance estimation, US2009310715 proposes to use “soft pilots”, which are “constant envelope” modulation symbols, for example Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK), with higher reliability than others. A two-pass G-RAKE is proposed, where the 1st pass employs MMSE G-RAKE to recover “soft pilots” and the 2nd pass uses them to estimate the impairment co-variance matrix and form combining weights to recover all the traffic data. However, it was found that the performance leaves room for further improvement. So far this desire remains very challenging to be met.